Title: Assessment of Dagaa (Rastrineobola argentea) stocks and effects of environment in Lake Victoria, East Africa.

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dagaa; Lake Victoria; abundance;


The abundance of dagaa (Rastrineobola argentea) in Lake Victoria has been reported to fluctuate over the long period. Several studies have accounted for this fluctuation being caused by predation, natural mortality and fishing pressure. This study examines the possible influence of climatic (rainfall, temperature and wind stress) and physicochemical (chlorophyll a and secchi depth) variables on dagaa abundance in Lake Victoria. Data for climatic, physicochemical and abundance recorded 1999 – 2014 were correlated and their trends examined. Analysis was done separately in three depth strata, deep (>40m), coastal (20 – 40m), inshore (< 20m), and two gulfs, Emin Pasha and Speke Gulfs. Population growth parameters of the species were also estimated in FiSAT II. The highest concentration of dagaa were found in the inshore shallow waters ≤40m, excluding the gulfs. Wind stress has significantly increased in Lake Victoria and was found to be the major driver that influenced the distribution of dagaa. In addition, the offshore waters are increasingly becoming important for dagaa. This could also be related to increase of wind stress in Lake Victoria that has increased the suitable habitat for dagaa. However, a limited correlation was found between other climatic and environmental factors with dagaa abundance, possibly due to the large volume of the lake with long flushing time, hence it is difficult to detect some effects with the short time series data available. The highest biomass densities of dagaa were found during the rainy season, which also coincided with the highest landing records. The estimated growth parameters (K, L∞) showed a continued decreasing trend towards smaller size. A big difference in the values of K and L∞ were also observed among the different localities of study, possibly due to the differences in exploitation rates and climatic conditions exhibited by these gulfs. Based on the yield per recruit plots, dagaa is not over exploited but matured at a smaller size compared to the previous studies. Dagaa being a short lived species, its commercial catch is possibly based on response to both environmental changes and exploitation rate. Although this study found a limited relationship with most of the climatic and physicochemical factors with dagaa, there is possibility that they play a great role in the complex ecosystem of Lake Victoria, hence important factors to be considered when planning management policies.

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